MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, instead of silicon dioxide, can reduce gate leakage. Because, however, such a dielectric may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics.
When making a CMOS device that includes metal gate electrodes, a replacement gate process may be used to form gate electrodes from different metals. In one variant of that process, polysilicon layers are removed to create first and second trenches. Both trenches are filled with a first metal layer. The first metal layer is then removed from the first trench. A second metal layer is then deposited on the first metal layer in the second trench, and on the high-k gate dielectric in the first trench.
In this variant of the replacement gate process, the high-k gate dielectric—within the first trench—may serve as an etch stop, when the first metal layer is removed from the first trench. When the high-k gate dielectric performs an etch stop function, the process for etching the first metal layer may damage the underlying dielectric, which may adversely affect the performance and reliability of transistors that include the high-k gate dielectric.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a high-k gate dielectric and a metal gate electrode. There is a need for a replacement gate process for making such a device that does not require the high-k gate dielectric to serve as an etch stop, when removing a metal layer from its surface. The method of the present invention provides such a process.
Features shown in these figures are not intended to be drawn to scale.